If you dig deeper and follow the associated links, you learn a few interesting things.
These numbers are based on having every meal and snack at/from home BUT they are not based on everything being made from scratch, esp. the Thrifty Plan! So, they include purchased breads, box meals, canned soups, processed foods, etc.
The numbers are also based on the MyPyramid food standards for servings, food group distribution, etc. I'm guessing a lot of people here on MDC (me included) don't buy into the standard "American diet", which changes things considerably.
Most importantly, the price numbers for foods come from Nielson Homescan data (now NCP) and represent average costs people actually paid for food. Not everyone uses coupons, shops loss leaders, meal plans, etc. so that can make a huge difference.
All that said...right now we're sitting at about $320 a month for a family of 4: 2 adults, a 16mo, and a 6yo. This doesn't include a bulk beef purchase we'll be getting in a few weeks, or the hog we'll purchase in several months. Even averaging those out over the next year, I'd say we'll still be between thrifty and low-cost plans. We shop sales, use doubled/stacked coupons, meal plan, maintain a hefty pantry, and generally do whatever we can to keep costs down.
Specialization is for insects. -- Lazarus Long